1. Field of the Invention
The present invention relates to the field of solenoid actuators.
2. Prior Art
Solenoid actuators, particularly in the form of linear actuators, are very well known in the prior art in various forms and for various applications. Generally such actuators consist of a stationary magnetic member and a moveable magnetic member forming a magnetic circuit having a minimum reluctance when the moveable magnetic member is in the actuated position and a larger reluctance when the moveable member is in the unactuated or extended position. In that regard as used herein, the words moveable and stationary are used in a relative sense and are meant to include applications wherein the moveable member, as referred to herein, is fixed to some frame of reference and the stationary member moves with respect thereto, as well as applications wherein both members move with respect to some fixed reference.
Solenoid actuators may be of the latching type or the non-latching type. Latching actuators in general normally will actuate and latch in the actuated position mechanically or magnetically when power is applied, and will remain latched until power is again applied and/or applied and removed in the same or some altered form, causing the same to then unlatch. Non-latching actuators, on the other hand, normally remain actuated only so long as power is applied. In that regard, the present invention generally relates to the field of non-latching actuators, and accordingly latching actuators will not be further discussed herein.
Actuators of the non-latching type may be operated either from a DC power source or an AC power source. In the case of a true DC power source such as a car battery or the like, eddy currents are only induced in the magnetic material during turn on and turn off of the solenoid, and not during the steady state on or off conditions. Accordingly, the heating of the magnetic material in the stationary and moveable members is generally negligible because of the very low duty cycle of the switching conditions and accordingly both members may be fabricated of solid magnetic (and electrically conductive) materials of substantially whatever size may be appropriate for the application without using laminated structures or other provisions to minimize eddy current losses. In solenoids intended for AC operation however, substantial eddy currents can be generated in the electrically conductive magnetic materials of the stationary and moveable member, which heating in many cases would be excessive unless laminated structures are used, especially for the normally larger stationary members. In the case of relatively small AC solenoids, special structures, typically of sheet magnetic material, may avoid the need for lamination, though such structures are in general not suitable for solenoids having higher mechanical energy output capabilities.
Also in the case of AC solenoids, the voltage in the coil and thus the current through the coil is constantly swinging through plus and minus extremes. Since the flux in the magnetic circuit is normally proportional to the current in the coil (for a fixed magnetic gap), and the force is proportional to the square of the flux density between the stationary and the moveable member, the actuating force is highest at the current extremes and rapidly diminishes as the current diminishes on each cycle. The net result is that the magnetic holding force of the solenoid while actuated has an average value plus a time variation between zero and twice the average value at twice the frequency of the excitation. Thus AC solenoids will tend to hum at twice the excitation frequency (120 hertz for 60 hertz excitation) and will actually chatter by varying amounts if a return spring or other mean starts to pull the moveable member away from the stationary member during a portion of each cycle when the flux density and thus the holding force is too low to adequately overcome the spring or other mechanism. Thus unless a special provision is made therefore, an AC solenoid may loudly chatter at a frequency equal to twice the excitation frequence. Similarly a DC solenoid operated on AC power will chatter, and may overheat on AC power.
A typical method of avoiding chatter in prior art AC solenoids is to "shade" part of a pole face between the stationary and the moveable magnetic members. Such shading consists basically of dividing the pole face into two portions, one of which is provided with a relatively low resistance conductor therearound. The net effect of the shaded pole is that the change in flux therethrough induces a current in the conductor which opposes such change in flux, causing the AC flux through the shaded pole to lag the AC flux through the portion of the pole which is not shaded. Thus the total flux in the working gap between the stationary member and the moveable never goes to zero, one pole portion always having a substantial flux density whenever the other pole portion has a low or zero flux density. By proper design the zero magnetic force at twice the excitation frequency which would occur in an unshaded configuration can be raised to a minimum force level equal to or exceeding a spring or other force tending to separate the moveable member and the stationary member, thereby avoiding the chatter. Shading however does tend to increase the cost, the heat generated by an AC solenoid and the required size thereof.
Another method of avoiding chatter in AC solenoids is to rectify the AC voltage, typically by a fullwave rectifier, and to drive the solenoid coil with the rectified voltage. Still the coil voltage will go to zero twice per excitation cycle, and accordingly in such instances it is common to provide a storage capacitor on the output of the fullwave rectifier to maintain adequate coil current during periods of zero or near zero excitation voltage. In that regard, particularly when using a DC drive or a rectified AC drive for solenoid power, normally the minimum working gap between the stationary member and the moving member is kept relatively significant so that the retentivity of the magnetic components will not inadvertently retain sufficient field strength to keep the solenoid latched when the excitation is removed therefrom. While the fullwave rectifier and storage capacitor work well and do not really compromise the solenoid design, they add size and expense in themselves, with particularly the capacitor contributing to a reduced life and reliability of the system.
The purpose of the present invention is to maintain simplicity and small size in the design and construction of the solenoid while at the same time minimizing the drive circuitry therefor to still allow operation on an AC excitation without chatter or inadvertent latching of the solenoid.